The present invention relates generally to the field of structural pier devices which function as footings or structural supports for walls, platforms, towers, bridges, building foundations and the like, and more specifically to the improved construction of devices known as “helical anchors,” “push piers” and the like, which are utilized for such purposes.
The foundations of many structures, including residential homes, commercial buildings, bridges, and the like, have heretofore conventionally been constructed of concrete slabs, caissons and footings upon which the foundations walls rest. These footings, which are typically constructed of poured concrete, may or may not be in contact with a stable load-bearing underground soil structure, and the stability of the foundation walls, and ultimately the entire structure being supported, depends on the stability of the underlying soil against which the footings bear.
Oftentimes the stability of the soil, particularly near ground surface, can be unpredictable. Changing conditions over time can dramatically affect the stability of the underlying soil, thereby causing a foundation to move or settle. Such settling can cause cracking and other serious damage to the foundation walls, resulting in undesirable shifting of the supported structure, and consequent damage to windows, doors and the like. This ultimately affects the value of the building and property upon which the building is situated.
In some situations, it has been found that the soil may simply be too unstable to cost effectively utilize concrete footings as the foundation for new construction. In other situations, existing concrete foundation walls have settled, causing damage and requiring repair. In still other situations, such as in some foreign markets, the shortage of concrete and abundance of residential and commercial construction has limited the use of poured concrete footings altogether. All of the above has led to the development and advent of alternative structural pier devices, such as the screw-in “helical anchor,” and the “push pier,” which are the subject of the present invention.
The use of screw-in helical anchors have become increasingly common for use as footings or underpinnings in new building construction, as well as for use in the repair of settled and damaged footings and foundations of existing buildings and other structures. Typically, in new construction, a plurality of such helical anchors are strategically positioned and hydraulically screwed into the ground to a desired depth where the underground stratum is sufficiently stable to support the desired structure. Once in place, the anchors are tied together and all interconnected by settling them within reinforced concrete. In a similar manner, such helical anchors are often positioned along portions of settling and damaged foundation walls of a structure, and utilized to repair the structure by lifting and supporting the settling foundation.
Exemplary systems utilizing Helical anchors or underpinnings of this type are disclosed in U.S. Pat. Nos. 5,011,336, 5,120,163, 5,139,368, 5,171,107, 5,213,448, 5,482,407, 5,575,593, and 6,659,692. The helical anchors in these systems will typically include at least one helical plate or flight welded to a drive shaft or column. The shaft and helical flights are generally constructed of a non-corrosive material, such as galvanized steel, to prevent deterioration of the anchor over time. Typically, the steel utilized will be a commercially available grade of about 0.18% carbon by weight, with a yield and tensile strength in the range of about 40,000-55,000 psi.
By way of example, and depending on the application, a standard round shaft starter section of a helical anchor may consist of a round hollow hot or cold rolled welded steel tubular shaft 2⅞″ thru 7.0″ O.D. typical, with one or more steel helical flights or plates of 6″-14″ in diameter welded at spaced intervals thereto. The helical flights typically range in diameter with the smaller diameter flight nearer the bottom of the drive shaft to ensure that the load-bearing surface of each helix partially contacts undisturbed soil upon insertion into the ground. The pitch of the steel flights may range from 3″-6″, and the starter section will have a pointed lower tip, such as by cutting the tip at a 45 degree angle.
Depending upon the application and depth required for reaching bedrock or other suitably stable strata to support the intended structure, multiple extension shafts also formed of hot or cold rolled steel, which may or may not include additional helical flighting, may be coupled to the starter shaft and each other, as needed. Heretofore, such coupling has been accomplished with the use of separate tubular coupling inserts having an outer diameter slightly smaller than the inside diameter of the extension and starter sections. Others have swelled one end of a shaft so as to form a female coupling for receiving an adjoining shaft. Such couplings are pre-drilled with multiple bolt holes that align with corresponding bolt holes in the adjoining ends of the starter and extension shafts. Bolts received through the aligned openings of the shafts and couplings act to secure the adjoining sections together.
Helical anchors of this type are generally torque-driven to bedrock, or to equal load-bearing strata which attains the installing torque that correlates to the required load-bearing capacity. As required load-bearing capacities increase, so does shaft and flight diameters, depth of installation, and consequently the required torque to install the anchors. As a consequence, it has been found that the greater torque generated at increased depths of installation causes coupling failures between the adjoining shaft sections. At or near the coupling joints, the pre-drilled holes in the shafts and inserts begin to tear laterally under excessive applied drive torque, thereby loosening and weakening the bolted joints, and ultimately causing catastrophic failure many feet below ground level. This is particularly true where the walls of the shafts are swelled and consequently thinned to form coupling ends. In other instances, excessive torque will lead to failure of the welded seams of the tubular shafts themselves, which also begin to split, thus causing further failure and weakening of the anchoring system. While the aforementioned conventional coupling system is adequate in applications requiring light to medium load-bearing capacities, it has proven to be insufficient for applications requiring increased load-bearing capacities and installation torque.
In addition to the above, the conventional coupling method utilizing coupling inserts is cumbersome to employ in that it includes multiple components, and is labor intensive and costly to implement. To couple adjoining drive shafts, a coupling insert must first be inserted within one shaft and bolted thereto utilizing a minimum of two (2) bolts. Then the adjoining shaft must be properly positioned over the remainder of the coupling insert and bolted thereto with a minimum of two (2) bolts. At each joint, a minimum of four (4) bolts are necessary to couple adjoining drive shafts together (2 for each shaft).
The use push piers have also become increasingly popular as an alternative structural pier device. Push piers, however, are most commonly used in situations involving foundation rehab construction, where an existing foundation has sunk or shifted for some reason, and needs repair or reinforcement. Unlike helical anchors, push piers are not rotated into the ground, but are rather hydraulically jacked straight into the ground with the use of a hydraulic press that is positioned adjacent to the failing foundation. In foundation rehab construction, push piers are often deemed preferable over helical anchors because they do not have the helical flights, and can therefore be positioned closer to the failing foundation, at a lower angle of incidence thereto.
Like helical anchors, conventional push piers are typically constructed from a steel tubular member having a yield and tensile strength on the order of about 40,000-55,000 psi. The lead section has a ground penetrating member or friction collar welded to one end of the main tubular shaft section. The opposite end of the main shaft terminates in an open end to form a female coupler that is adapted to receive a corresponding male coupler of an adjoining extension shaft. The friction collar is typically constructed of the same material as the main shaft section, and may be constructed with an open or closed (typically blunt) end, depending on the soil conditions and needs of a specific project.
Similar to helical anchors, push piers are driven deep into the ground until bedrock or other suitably stable strata is reached which is sufficient to support the failing foundation. As the push pier is driven deeper, multiple extension shafts are required to extend the length of the push pier. Each extension shaft is typically about three (3) feet in length, and is constructed of a steel tubular member having strength characteristics similar to that of the lead section of the push pier.
As noted above, one end of an extension shaft forms a male coupler or pin of reduced diameter which is adapted to be inserted within the open terminal end of the push pier lead or other extension shaft. To form such a male coupling pin, another tubular member of smaller diameter is typically inserted within one end of the tubular extension shaft. One or more holes are then drill through the outer tubular member adjacent the smaller inserted tube, and the telescoping tubular members are then welded together through such holes. This creates a clean, flat interface between adjoining sections of the push pier.
Unfortunately, this construction of the male coupler forms a weakened joint due in part to the fact that the material from which the inner tubular member is constructed is often of lower grade steel and of thinner wall construction than the outer shaft section. More importantly, however, the inner/outer tubular connection in this type of conventional construction creates a relative sloppy, loose fit to facilitate insertion and welding of the inner tubular member within the outer tube, which results in an overly loose, weak joint at each adjoining section of the pier. Also, since both the lead and extension shafts are typically constructed using an ERW (electric resistance weld) manufacturing process, like conventional helical anchors, a longitudinal seam is created, thereby creating another weakened area which is subject to rupture under extreme loads.
While push piers do not experience the torsional load of a helical anchor upon installation, such pier devices oftentimes experience significant lateral forces due to shifting soil structures caused by settling of the soil, frost, drainage, etc., and to a certain extent, the weight of the foundation itself. Such lateral forces can unduly stress the longitudinal shaft seams, but more importantly, the weak joints between adjoining sections can bend, deflect and even shear off, thus causing failure of the supporting structure altogether. In order to prevent such an occurrence, project structural engineers oftentimes require a secondary outer continuous steel tubular member to be slid over the outside of the extended push pier shaft (particularly near the ground surface) so as to cover the weaker joints between extension shafts, and add lateral strength to the push pier. This obviously results in a relatively significant increase in material and labor cost to the project.
It is therefore evident that there is a distinct need for an improved means of coupling the drive shafts of structural pier devices, such as helical anchors and push piers, so as to withstand the significant forces exerted on such coupling devices in applications requiring increased load-bearing capacities, increased drive torque for installation and/or lateral forces experienced during and after installation. It is also evident that the present coupling methods for such devices are cumbersome, time consuming to implement, and would benefit through simplification. It is with these objects in mind that I have developed an improved shaft construction for such structural pier devices which has an integrally-fabricated drive shaft coupling capable of withstanding increased torque under applications requiring significant load-bearing capacity and/or significant lateral forces exerted thereon.